Looking ill-at-ease, as though her quest for an elusive 22nd grand slam title was weighing heavily on her shoulders, Williams struggled with her serve and sprayed balls wide on both sides of the court. She served three double faults in the fifth game of the first set.

It was Williams's first match since she lost the French Open final to Garbine Muguruza this month and the American should have had few problems against Macedonian-born Sadikovic, ranked 148th in the world and with little main tour experience.

By turns urging herself on and telling herself off, Williams eventually secured match point when her opponent just missed a lob and sealed victory after challenging a wrong call when her shot landed on the far baseline.